In drilling wellbores through subsurface formations, e.g., for extraction of materials such as hydrocarbons, it is known in the art to directly or indirectly mount a rotating control device (RCD) on the top of a wellhead or a blowout preventer (BOP) stack. The BOP stack may include an annular sealing element (annular BOP), and one or more sets of “rams” which may be operated to sealingly engage a pipe “string” disposed in the wellbore through the BOP or to cut the pipe string and seal the wellbore in the event of an emergency.
The RCD is an apparatus used for well operations which diverts fluids such as drilling mud, surface injected air or gas and other produced wellbore fluids, including hydrocarbons, into a recirculating or pressure recovery “mud” (drilling fluid) system. The RCD serves multiple purposes, including sealing tubulars moving in and out of a wellbore under pressure and accommodating rotation and longitudinal motion of the same. Tubulars can include a kelly, pipe or other pipe string components, e.g., parts of a “drill pipe string” or “drill string.”
Typically, a RCD incorporates three major components that work cooperatively with one another to hydraulically isolate the wellbore while diverting wellbore fluids and permitting a pipe string (e.g., a string) to rotate and move longitudinally while extending through the RCD. An outer stationary housing having an axial bore is hydraulically connected to the wellhead or BOP. The outer stationary housing can have one or more ports (typically on the side thereof) for hydraulically connecting the axial bore of the housing to return flow lines for accepting returning wellbore fluids. A bearing assembly is replaceably and sealingly fit within the axial bore of the outer housing for forming an annular space therebetween. Wellbore fluids can travel along the annular space and can be redirected out the side ports to the recirculating or pressure recovery mud system.
The bearing assembly comprises a rotating inner cylindrical mandrel replaceably and sealingly fit within a bearing assembly housing. An annular bearing space is formed between the rotating inner cylindrical mandrel and the bearing assembly housing for positioning bearings and sealing elements. The bearings permit the mandrel to rotate within the bearing assembly housing while the sealing elements isolate the bearings from wellbore fluids.
In deep water offshore applications, the RCD can be installed either below or above a marine riser tensioning ring. The marine riser tensioning ring is supported below an offshore drilling unit (“rig”) platform by tension cables. Installation of the RCD below the tensioning ring requires the outer stationary housing of the RCD to be incorporated into and during the manufacture of the marine riser.
Installation of the RCD below the tensioning ring can be advantageous because the RCD is manufactured specifically for the particular riser being used and thus is secured and stationary. The RCD, as part of the marine riser, is typically submerged and thus is not subjected to types of movement experienced by the rig platform and associated equipment above the water surface. The submerged RCD is substantially immune from movement such as heave and rotational movements caused by the ocean tides and currents. Further, because the return flow lines from the RCD are located below the tensioning cables of the rig platform, there is only very limited risk of the tensioning cables becoming entangled with the return flow lines.
However, because the outer stationary housing of the submerged RCD is manufactured as part of the riser system, the RCD cannot be used for any other application other than for the particular riser for which it was manufactured. The RCD thus becomes a component of an individual marine riser system that cannot be used in any other marine riser system. This further requires the RCD manufacturer to produce the RCD with all possible flow lines that the RCD may need to incorporate during its operational life as part of the particular marine riser system.
It is important to note that a submerged marine RCD is also subject to conditions that are not typically associated with RCDs used on land or above the water surface in marine drilling. Exposure to hydrostatic pressure, for example, necessitates the use of RCD specific and typically non-API (American Petroleum Institute) standard couplings. Such requirements further increase manufacturing and operational costs associated with using a RCD installed below the riser tensioning ring.
Another disadvantage of a submerged RCD is the limited access to the RCD. One of the most common sources of premature failure of RCDs is a result of the failure of the bearings between the bearing assembly housing and the mandrel. Failure of the bearings in an RCD below the tensioning ring requires the complete shutting down of well operations, closing all the sealing elements of the BOP and withdrawal of the riser system from the water to gain access to the failed and submerged RCD, and the removal thereof from the riser system. Repairs to the submerged RCD can be substantially time-consuming and thus what is known as “non-productive time” (NPT) increases significantly, driving up operational cost of the particular well affected by the failed RCD.
Although RCDs installed above a marine riser tensioning ring minimize the disadvantages mentioned above, simply installing a conventional RCD above the tensioning ring will not significantly reduce the NPT when operational equipment requires maintenance. It is still necessary to remove at least part of the riser from the wellbore and remove the entire RCD from the riser system in order to repair the failed internal components.
Common to RCDs installed either above or below the tensioning ring, typical in-service time numbers in the tens to low hundreds of hours before some part of the operational equipment requires service or other attention including drill bit replacement or other downhole equipment such as motors, turbines and measurement while drilling systems. It is desirable that a RCD last as at least long as other drill string components and not be the reason drilling operations are interrupted so as to result in NPT. Further, existing retrieval techniques risk loss of conventional RCD components downhole. Such loss may require time consuming and expensive retrieval (“fishing”) operations to remove the lost components before drilling operations can resume.
There is a need for a rotating control device or rotating flow head that is easily accessible for repair and permits easy access to downhole tools requiring repair. There is also a need for a rotating control device that can be easily maintained and repaired on a rig platform to minimize NPT and minimize operational risk.